Harmonic distortion concerns are serious when the power rating of a variable frequency drive (VFD) load increases. Large power VFDs are gaining in popularity due to their low cost and impressive reliability. In many large power installations, current harmonic distortion levels achievable using traditional 12-pulse techniques are insufficient to meet the levels recommended in IEEE 519 (1992). In view of this, active techniques have been proposed that achieve much superior harmonic performance. However, these schemes are expensive and can create unwanted EMI.
In general, the characteristic harmonics generated by a semiconductor rectifier is given by:h=kq±1  (1)
where, h is the order of harmonics; k is any integer, and q is the pulse number of the rectifier (six for a 6-pulse rectifier). The per unit value of the characteristic harmonics present in the theoretical current waveform (assuming a rectangular current waveform spanning 120 electrical degrees) at the input of the semiconductor rectifier is given by 1/h. In practice, the observed per unit value of the harmonics is greater than 1/h. From equation (1), it is clear that increasing the pulse number from 6 to either 12 or 18 will reduce the amplitude of low order harmonics and hence the total current harmonic distortion.
For 12-pulse operation, there is a need for two sets of 3 phase AC supply that are phase shifted with respect to each other by 30 electrical degrees. Traditionally, this is achieved using a three winding isolation transformer that has one set of primary windings and two sets of secondary windings. One set of secondary winding is in phase with the primary winding, while the other set is phase shifted by + or −30 electrical degrees with the primary. This arrangement yields two phase-shifted supplies that allow 12-pulse operation as shown in FIG. 1. The main disadvantage of the scheme shown in FIG. 1 is that the isolation transformer is large in size, heavy and costly.
On re-examining the circuit of FIG. 1, it can be noted that one set of windings does not have any phase shift with respect to the primary windings. This is important because it allows one six-pulse rectifier circuit to be directly connected to the AC source via a balancing inductance and the other six-pulse rectifier via a half power rated isolation transformer. The inductance of the balancing inductor is matched to the leakage inductance of the half-power isolation transformer to assure good current balance to achieve 12-pulse operation.
The 12-pulse arrangement described above has been referred to as a hybrid 12-pulse configuration and is shown in FIG. 2. The phase shifting transformer feeding one of the two six-pulse rectifiers is sized to handle half the rated power [Tsuneo Kume, “Multi-Pulse Rectifier Circuit”, Japanese Patent P3591548, Sep. 3, 2004]. Similarly, the matching inductor is sized to carry only half the rated current. This arrangement results in the overall size of the transformer and matching inductor combination to be smaller and less expensive than the three winding arrangement.
Among advantages of the hybrid 12-pulse scheme are that size and cost of the hybrid 12-pulse configuration is much less than the 3-winding arrangement. 12-pulse operation is achieved with low total current harmonic distortion, typically less than 10% THD at rated load condition. Unlike the 3-winding method, in this method the current (instead of flux in the core) in the two bridges are combined at the source to cancel the low order harmonics. Leakage flux and winding mismatch problems do not occur.
However, the scheme shown in FIG. 2 still has some drawbacks. The impedance mismatch between the leakage inductance and the external matching inductance can never be accomplished for all operating conditions because the leakage inductance is a function of current through the transformer while the external inductance is in the form of self inductance, which is constant until its rated current value. In order to achieve THD levels of less than 8%, an input AC line inductor may need to be used. The arrangement of FIG. 2 cannot be used where voltage level translation is needed.
The phase shift necessary to achieve multi-pulse operation can also be achieved by using autotransformers. Autotransformers do not provide any isolation between the input and output but can be used to provide phase shift.
Autotransformers are typically smaller compared to regular isolation transformers because they do not need to process all of the power. The majority of the load current passes directly from the primary to the secondary terminals and only a small amount of VA is necessary for the phase-shift processed by the autotransformer. This makes them small, inexpensive, and attractive for use in multi-pulse systems.
The autotransformer technique can be broadly classified into three distinct groups. The first is the traditional group that aims at providing the needed +/− 15 degree or 30 degree phase shift using either fork or polygon type autotransformers. The second group consists of employing autotransformers to convert the input 3-phase supply to a balanced 6-phase output, bearing a 60 degree phase difference among the six outputs. The last group consists of utilizing autotransformers to provide the missing parts of the typical discontinuous waveform seen in 6-pulse rectifiers, resulting in a topology that can be said to consist of asymmetrical rated diode rectifier bridges. A sample representative for each of the three groups of autotransformers used in multi-pulse application is discussed next.
For a +/−15 degree Phase Shift Autotransformer, when parallel rectifiers are used as in multi-pulse techniques, it is important to maintain sharing of current among the multi-pulse rectifiers. If current sharing is compromised, then the amplitudes of lower order harmonics between the two rectifiers in a 12-pulse scheme will not cancel completely and this will result in poor harmonic performance. By electrically isolating one rectifier from the other either by using three-winding isolation transformer or by using half-power isolation transformer, in the two schemes discussed earlier, acceptable 12-pulse performance was possible. However, when autotransformers are employed, such isolation is lost and current from one set of phase-shifted windings can flow into the other set, thereby compromising the equal distribution of current between the phase shifted sets of windings. One way to force the rectifiers to share correctly is to introduce an inter-phase transformer (IPT) in between the outputs of the two diode-rectifier units as shown in FIG. 3. Zero-sequence blocking transformers (ZSBT) in between the rectifiers and the phase shifted outputs of the autotransformer are needed to reduce non-characteristics triplen harmonics from flowing into the AC system. The autotransformer of FIG. 3 has phase shifted outputs of ±15°. The addition of IPT and ZSBT helps in reducing non-characteristics low order harmonics from flowing into the AC system but adds to cost and size of the total system.
A 3-phase to 6-phase Fork-type Autotransformer is shown in U.S. Pat. No. 4,255,784. In this patent, the voltage imbalance with many autotransformer base topologies is overcome by adopting a 3-phase to 6-phase converter that yields a balanced 6-phase output that has the phases phase shifted by 60 electrical degrees between each other. By doing this, the voltage imbalance issue is resolved thereby not requiring the use of IPT and ZSBT.
The voltage imbalance problem associated with many autotransformer schemes can also be overcome by adopting a 3-phase to 9-phase converter that yields a balanced 9-phase output that has the phases phase shifted by 40 electrical degrees between each other. By doing this, the voltage imbalance issue is resolved thereby not requiring the use for IPT and ZSBT. One such scheme has been proposed in U.S. Pat. No. 5,455,759, which results in 18 pulse operation.
However, the 9-phase output has a few drawbacks. The rectified output voltage from a balanced 9-phase output is about 1.14 times higher (14% higher) than that obtained from a 3-phase 6-pulse AC to DC rectifier. This requires modifying the basic 3-phase to 9-phase converter using more windings and alternate paths to provide the needed step down action necessary to use the resulting topology on an existing 3-phase AC to DC rectifier system. Many topological modifications including the one in the '759 patent have been described in patents to overcome this issue, including U.S. Pat. Nos. 5,619,407; 6,249,443; 6,335,872; 6,191,968 and 6,525,951. However, the extra stub and teaser windings add cost and complexity to the structure.
The second important issue observed in all 3-phase to 9-phase autotransformer schemes is the sudden change in current from one conducting pair to the other every 40 electrical degrees. The situation is amplified since most autotransformers do not have enough leakage inductance to slow the transition resulting in high di/dt across the rectifier diodes. This situation has been studied and statistical records have shown that standard rectifier grade diodes are vulnerable to premature failure. The sudden change in current also reflects on the input lines, making the current have sharp edges with quick transitions. Such abnormalities deteriorate the harmonic performance and so there is inherently a need to add large inductance to smoothen the current waveform. In such topologies, the input inductor can be as high as 0.075 pu, see the '968 and '951 patents.
For 3-phase to 9-phase Polygon type Autotransformers applications, many autotransformers employ either delta fork or wye fork type of windings. Such autotransformer configurations use stub and/or teaser windings resulting in a structure where the overall size and rating of the autotransformer is higher than the optimal value. Use of stub windings typically results in poor utilization of the core and involves more labor to wind the coils.
Polygon type autotransformers are preferred over stub type autotransformer from size and core utilization points of view. Such an autotransformer has been put forward in U.S. Pat. Nos. 6,249,443 and 6,335,872. The kVA rating of the 3-phase to 9-phase autotransformer for 18-pulse operation proposed in the latter is reported to be only 34% of the rating of a standard 4-winding isolation transformer. However, in the schemes presented in these patents, the problem of low inductance, high di/dt of rectifier current, and winding manufacturing complications persist. In fact, in these schemes, each core is required to have five separate windings with multiple connections that need to be brought out to make appropriate connections with other phases. The labor involved and the complexity of the windings are daunting.
A more recent polygon configuration referred as the irregular polygon is given in U.S. Pat. No. 7,274,280. However, this structure also suffers from complexity in winding, has higher losses due to current flowing through more number of windings, and still needs a fairly significant input impedance to smooth the sharp edges due to the sudden change in current associated with autotransformers.
In the schemes discussed thus far, the intent was to provide equal and balanced output voltages to rectifier diodes so that current is equally shared and the total harmonic distortion is low. A different school of thought, where the missing portions, that form the discontinuity in typical 6-pulse rectification, is added to the input current waveform by employing asymmetrically sized rectifiers and autotransformers.
One such scheme has been proposed in G. R. Kamath, D. Benson, and R. Wood, “A Compact Autotransformer based 12-Pulse Rectifier Circuit”, in IECON 2001, pp. 1344-1349. This scheme aims at providing asymmetrical conduction to result in 12-pulse operation. The main power is directly conducted from the mains and the autotransformer is used only to provide auxiliary current to fill the missing parts in a traditional six pulse operation.
This idea has been extended to 18-pulse operation by the authors in G. R. Kamath, D. Benson, and R. Wood, “A Novel Autotransformer based 18-pulse Rectifier Circuit”, in Applied Power Electronics Conference and Exposition”, 2002; and U.S. Pat. No. 6,396,723. Asymmetrical conduction of the rectifiers forms the basis of these. For example, in the 12-pulse scheme, the main rectifier carries 75% of load current and the auxiliary rectifier carries the remaining 25%. Similarly in the latter article, the combined current drawn by the auxiliary rectifiers is 33% of the total load current, with the main rectifier carrying the remainder.
For retrofit applications and applications that have built-in 12-pulse or 18-pulse rectifier units of equal rating, the schemes presented above are not easily applicable, because the main diode rectifier module needs to be rated differently compared to the auxiliary rectifier modules. Further, in the tests reported on such schemes, the input inductor plays a significant role in smoothing the unbalanced current flow and improving the overall harmonic distortion.
From the discussion on autotransformers thus far, some important shortcomings of the autotransformer based topology are summarized. The leakage and magnetizing inductances of many autotransformers in the market is far lower than that in isolation transformers. Powering up an autotransformer typically results in an inrush current that is much higher than that observed in systems with isolation transformer. This requires careful fuse selection and coordination so that nuisance trips are avoided and fuse protection is still available. In all the 18-pulse autotransformer methods, the change of current from one conducting diode pair to the other is quick. Though the rms current rating may not exceed the current rating of the diode, attention should be given to the di/dt of the current through the diodes. One solution is to use additional inductors in between the autotransformer and the input rectifier to lower the di/dt. This makes the overall scheme bulky and expensive. The rectangular current through the windings also increases losses, prompting the need to use fans to keep the size of the transformer small. Due to the sudden change in current and lack of sufficient leakage inductance in autotransformers, such topologies require significant input impedance to smooth the current and reduce the overall input current distortion. All the autotransformer configurations discussed here do not operate well without a significant amount of input inductance ahead of the autotransformer. Autotransformer techniques utilize complex winding structures, either of the stub-type or the polygon type. These transformers are labor intensive to manufacture and result in poor core utilization. Even with the polygon types, the number of windings per core is large and is a labor intensive winding process. Autotransformer topologies that convert a 3-phase system to a 9-phase output create an aberration in the DC bus ripple content of a VFD. When one or two of nine output phases has a bad rectifier, the increase in DC bus ripple is hardly noticeable and this reduces the chance for detection of failure. The power flow is now shared by existing rectifiers that can eventually fail. Given the above shortcomings, it is clear that there is room for improvement in multi-pulse rectification schemes.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.